THE FIRST ANIMALS
According to molecular clock data, the separation of the Animal Kingdom
from the Plant Kingdom occurred 1609±60 Ma (2004HedgesSB_ShoeJL). Molecular estimates indicate that the last common
ancestor of multicellular representatives of the fauna (Metazoa) was
Neoproterozoic, most likely younger than 800 million years (2015PisaniD_LiuAG).
Biomarkers of metazoan living systems identified in sedimentary deposits
indicate their presence in the seas since the Cryogenian period
(2018ZumbergeJA_SummonsRE), which began around approximately 720 Ma (2023CohenKM_CarN).
Multicellular animals were part of early Ediacaran ecosystems and are
found below layers dated to 632.5 ± 0.5 Ma (2007Yin_LHuJ). The
oldest fossil of a multicellular organism resembling a sponge (Porifera) is
about 600 Ma (2015YinZ_TafforeauP). The last Neoproterozoic
glaciation (about 582 Ma) coincides with the appearance of
complex organisms in the fossil record (2007BowringSA_AllenPA). Evidence of
burrowing by any animal larger than 1 cm is absent from rock layers formed
before 575 Ma (2002ErwinDH_DavidsonEH). A diverse assemblage of
fossilized bodies and tracks of bilateral organisms (Bilateria) have been found
in deposits ranging in age from 558±1.0 to 555.3±0.3 Ma (2000MartinMW_KirschvinkJL; 2003GrazhdankinDV). The oldest protostomes
(Protostomia) like mollusks with shells were given the name Kimberella
(2007FedonkinMA_IvantsovAY).
Around 543 Ma, a complex community of soft-bodied
bilaterians appears in the fossil record (1995GrotzingerJP_KaufmanAJ). The
tracks of the first burrowing creations of the worm Treptichnus pedum are dated
to the period 542-541 Ma (2007BowringSA_AllenPA). Perhaps 540 Ma, Saccorhytus coronarius, the founder of the line of
deuterostomes (Deuterostomia), saw the light of day (2017HanJ_HuangH). In the
group of bilaterally symmetrical animals, the phylum Chordates
(1995ChenJY_ZhouGQ) emerged 525 Ma. This lineage laid the
foundations for the creation of an internal skeleton and joints, in one of
which ligamentum capitis femoris (LCF) was later formed.
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| Upper Vendian, Mezen Formation (about 555 Ma). Slab with imprints of transversely segmented organisms from the extinct type Proarticulata, showing traces of movement and feeding (fragment). Exposition of the Orlov Paleontological Museum (Moscow); photo by the author. |
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| Ovatoscutum concentricum Glaessner et Wade. Vendian, Winter Coast of the White Sea. Collection of the Orlov Paleontological Museum (Moscow); photo by the author. |
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| Brachiopods. Collection of the Orlov Paleontological Museum (Moscow); photo by the author. |
 |
| Scheme of structure (A) and reconstruction (B) of the most ancient chordate Haikouella, early Cambrian, approx. 539 million years. Exposition of the Orlov Paleontological Museum (Moscow); photo by the author. |
References
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Pisani D, Liu AG. Animal evolution: only rocks can set the clock. Current Biology. 2015;25(22)R1079-81. [cell.com]
Zumberge JA, Love GD, Cárdenas P, Sperling EA, Gunasekera S, Rohrssen M, Grosjean E, Grotzinger JP, Summons RE. Demosponge steroid biomarker 26-methylstigmastane provides evidence for Neoproterozoic animals. Nature Ecology & Evolution. 2018;2(11)1709-14. [nature.com]
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Yin Z, Zhu M, Davidson EH, Bottjer DJ, Zhao F, Tafforeau P. Sponge grade body fossil with cellular resolution dating 60 Myr before the Cambrian. Proceedings of the National Academy of Sciences. 2015;112(12)E1453-60. [pnas.org]
Erwin DH, Davidson EH. The last common bilaterian ancestor. Development. 2002;129(13)3021-32. [journals.biologists.com]
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Гражданкин ДВ. Строение и условия осадконакопления вендского комплекса в Юго-Восточном Беломорье. Стратиграфия. Геологическая корреляция. 2003;11(4)3-23. [repository.geologyscience.ru]
Fedonkin MA, Simonetta A, Ivantsov AY. New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications. Geological Society, London, Special Publications. 2007;286(1)157-79. [lyellcollection.org]
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Han J, Morris SC, Ou Q, Shu D, Huang H. Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China). Nature. 2017;542(7640)228-31. [nature.com]
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